This invention relates to the field of photographic media, physical development photography, and to various uses such as photograpic film, paper, and printing plates.
Photographic processes utilizing "physical developers" comprising a solution of metal ions and a reducing agent for these metal ions have been around for many years. In these processes an image pattern of physical development nuclei is formed such as by exposure of a photographic element comprising a photosensitive layer which upon exposure generates physical development nuclei. The photographic element containing the image pattern of physical development nuclei is then contacted with a physical developer to produce a metallic image in those areas where the physical development nuclei catalyze the selective deposition of metal.
Two types of physical development processes have achieved commercial success. The first is the diffusion transfer type of physical development which may more accurately be termed "solution physical development" wherein a solvent is added to the developer to thereby dissolve the silver halide in the unexposed portions of a silver halide copy medium thereby allowing the silver ions to be utilized for image formation on suitable physical development nuclei. These nuclei may be either in a separate layer on the same sheet as the silver halide layer or on a separate transfer sheet as is well known in the Polaroid process. A second physical development process which has achieved some commercial success is one wherein the metal ion solution is applied in a separate bath to the exposed photographic element and subsequently a reducing agent bath is applied to the element.
However, at the present time applicant is unaware of any commercial photographic processes utilizing a "unitary" physical developer comprising a solution of metal ions and a photographic reducing agent for these metal ions. A significant reason for this lack of commercial success with the unitary physical developer has been the lack of stability of this physical developer solution. A number of attempts have been made to overcome the instability of these physical developers by the addition of ionic surfactants, antifogging agents, and the like. See for example, U.S. Pat. Nos. 3,157,502 and 3,390,998, incorporated herein by reference. See also Jonker, et al. in Photographic Science and Engineering, vol. 13, 1969, pp. 38-44, also incorporated herein by reference, wherein a preferred unitary physical developer is discussed comprising a solution of ferrous ions, ferric ions, silver ions, a complexing agent such as citric acid, and an ionic surfactant as a stabilizing agent for the physical developer. By adjusting the concentration of the various components of this physical developer, relatively rapid rates of development have been achieved while maintaining a relatively stable physical developer. However, even with this preferred physical developer while the stability of the solution may be almost indefinite when sealed from the atmosphere and not in use, while being exposed to an oxidizing atmosphere such as air and during continual use this developer depending upon the concentration and activity, deteriorates within the matter of hours or at least a few days.
Two problems which are particularly critical in making printing plates by means of a unitary physical developer are (1) developer stability and (2) loss of fine detail or highlight portions of the printing plate. In the formation of printing plates it is particularly critical to have a physical developer which can deposit a large amount of metal compared to regular photographic applications such as film or paper. Thus, for example, for a useful printing plate it is necessary to deposit from about 0.1 - 20 grams of metal per square meter of printing plate to produce a useful plate wherein the image will accept ink and be able to undergo the abrasion of a printing press. A preferred physical developer for depositing this amount of metal and also the type of metal needed for achieving a long running printing plate is a unitary physical developer. Using this type of physical developer, a printing plate can be produced by contact with the developer for no more than 2 or 3 minutes. However, in order to achieve this type of physical developer it is necessary to utilize developers of such activity and concentration that the stability is lower than would be desired.
The problem of losing fine detail or highlight dot areas of the printing plate is particularly serious in trying to achieve high quality "halftone" or "continuous tone" printing plates using these physical development methods on aged photosensitive printing plates. These aged plates are ones which have been stored for a time period after manufacture. Thus, using a standard photographic test target, it is often impossible to obtain the 5% dots even in the 65 line screen areas. It is even more difficult to obtain these 5% dots in the 130 line screen areas. For high quality printing this type of loss makes the printing plate entirely unacceptable.
Applicant's invention overcomes these problems of the prior art by first in the case of the stability problem allowing applicant to utilize a unitary physical developer of lower activity or development rate which is inherently more stable than a developer of greater activity. Secondly, by utilizing applicant's invention it is now possible to produce printing plates having good fine detail and highlight dots such as shown in the 5% dot areas of a 65-130 line screen of an ordinary photographic test target. By utilizing an excess of physical development activator in the photographic element beyond what is needed to give good 5% dots in the 130 line screen area, it is possible to achieve the consistantly good results in the 5% dot areas which are necessary for a commercial printing plate for high quality printing. Additionally, 5% dot areas so produced wear well under the often severe abrasion and wear conditions of the printing press.